Pultrusion is a manufacturing process for producing continuous lengths of fiber reinforced plastic (“FRP”) structural shapes. Raw materials include a liquid resin mixture (containing resin, fillers and specialized additives) and reinforcing fibers. The process involves pulling these raw materials, rather than pushing as is the case in extrusion, through a heated steel forming die using a continuous pulling device. The reinforcement materials are in continuous forms such as rolls of fiberglass mat or doffs of fiberglass roving. The two ways to impregnate, or “wet out”, the glass are open bath process and resin injection. Typical commercial resins include polyester, vinyl esters, phenolics, and epoxy compounds. These resins usually have very long gel times and can be run in an open bath process wherein the reinforcing fibers are soaked in a bath of resin and the excess resin is scraped off by a series of preform plates and at the die entrance. As the wetted fibers enter the die, the excess resin is squeezed through and off the reinforcing fibers. The pressure rise in the die inlet helps to enhance fiber wet-out and suppresses void formation. As the saturated reinforcements are pulled through the die, the gelation (or hardening) of the resin is initiated by the heat from the die and a rigid, cured profile is formed that corresponds to the shape of the die.
For resin systems like polyurethanes, which have a fast gel time and a short pot life the resin injection process is used. In the injection process, the reinforcement materials are passed through a small closed box which is usually attached to the die or may be part of the die. The resin is injected, under pressure through ports in the box, to impregnate the reinforcement materials. Resin injection boxes are designed to minimize resin volume and resin residence time inside the box. There are a number of different resin injection box designs in the literature all of which have the common features of an angled or tapered design and the exit profile matching the shape of the die entrance.
Polyurethane pultrusion has struggled to gain market acceptance because it is not light stable, and therefore requires a protective coating. Pultruded polyurethane articles have proven difficult substrates to coat due to high surface hardness and low surface tension resulting from the presence of an internal mold release (“IMR”) at the surface.
Numerous attempts have been made by workers in the art to find a suitable coating and coating process for protecting polyurethane-based pultruded composites. Each coating required a primer to achieve the targeted adhesion specification. Pultruders prefer that no substrate pretreatment, such as solvent wash, flame or plasma treatment, be used, due to increased processing time and costs.
Therefore, a need exists in the art for a pultrusion coating which adheres directly to unprepared polyurethane pultruded composites, thus eliminating the need for substrate pretreatment.